Background Pressure Effects on Ion Velocity Distributions in an SPT-100 Hall Thruster

Abstract

Increased background pressure in vacuum chamber test facilities as compared to on-orbit operation has been shown to influence the operation of electric propulsion devices such as Hall thrusters. This study aims to elucidate the impact of pressure on the ionization and acceleration mechanisms in a stationary plasma thruster, model SPT-100 Hall thruster, using time-averaged and time-resolved laser-induced fluorescence velocimetry. The results are compared for the thruster operating at an applied 300 V (∼4.25  A), with vacuum facility background pressures ranging from 1.7×10^(−5) to 8.0×10^(−5)  torr. Time-averaged measurements reveal that, in general, an upstream shift in the position of the ionization and acceleration regions occurs as the facility pressure is increased above the nominal 1.7×10^(−5)  torr. Time-resolved measurements, implemented using a sample-hold scheme with 1  μs resolution, emphasize that similar acceleration profiles are present within the Hall thruster discharge channel regardless of background pressure. Measurements taken at 3.5×10^(−5)  torr, where the facility background neutral density is similar to the neutral density emitted from the thruster, unexpectedly show increased ion acceleration over the next highest pressure condition at 5.0×10^(−5)  torr. These results indicate a not-yet well defined balance of the impacts of neutral ingestion, classical and turbulent electron transport on thruster operation, and that the ratio of the background to thruster neutral density is a more relevant benchmark than background pressure alone when evaluating Hall thruster operation.

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